This invention relates to the use of certain nanoscale (i.e. less than 100 nm) particles (nanoadditives) in conjunction with cationic polymers to enhance conditioning and dewatering of digested and waste return activated sludge.
Approximately 10 to 25% of the overall energy use in water and wastewater treatment is utilized for various sludge handling processes. Sludge generated in wastewater treatment is typically thickened, digested, and then dewatered prior to offsite disposal. In some cases, the dewatered sludge is either incinerated on-site or further thermally dried and hauled off-site. The solids content of the thickened sludge is about 3 to 5%, which is ultimately increased to over 30% after sludge dewatering/drying activities. Various cationic polymers (e.g. quaternary ammonium) and/or conditioning aids (e.g. ferric chloride) are added to enhance the process/energy efficiency of sludge dewatering process.
Cationic polymers are reported to impact efficiency of sludge dewatering processes by i) neutralizing the negative charge of sludge solids to generate a sludge with higher percent solids, ii) reducing shearing of sludge in the dewatering (centrifuge, belt filter press) equipment, and iii) retaining/releasing the volatile fraction of sludge solids thereby impacting the auto-thermal characteristics.
The dewatered sludge is often hauled to an off-site location for ultimate disposal. The sludge hauling cost can constitute to over 50 to 70% of the overall sludge treatment/disposal cost. Increasing the % solids of the dewatered sludge (through polymer treatment) has a huge impact on sludge disposal cost. For example, increasing the percentage solids of the dewatered sludge from 25 to 30% can lower the sludge volume by 17% and lower the overall sludge handling cost by up to 12%.
The polymer-aided sludge dewatering is carried out using mechanical devices like belt-filter presses, as well as low, medium or high shear centrifuges. A recent survey by North East Biosolids and Residuals Association (NEBRA) indicated about 65% of the mechanical devices used in the United States are high shear centrifuges due to the higher percent solids (hence, lower mass) produced during dewatering by these devices.
While centrifuges produce high solids containing sludges (thereby lowering the sludge mass that requires disposal) recent studies have shown that high shear centrifuges release more bioavailable protein from sludge solids during the dewatering process. The bioavailable proteins are found to be a major source of odor causing compounds (e.g. volatile organic sulfur and volatile aromatic compounds) in the land applied sludges. Additionally, the energy demand for high shear centrifuges can be about an order of magnitude higher than that of dewatering using low shear or belt-filter press devices.
The type and dosage of polymer required for optimum dewatering in high shear or other dewatering devices is significantly impacted by the variability in sludge characteristics and dewatering equipment. Although polymers have been used for dewatering for several decades, and several hypotheses have been proposed on the role of polymers, to date the mechanism of polymer interactions for sludge dewatering has not been fully understood. A lower than optimum dose of polymer addition will generate a dewatered sludge with low percentage solids, and higher than optimum polymer addition may release more volatile fraction of the sludge solids.
Despite a large number of studies performed to date on polymer-aided sludge dewatering, one major area that has not been adequately addressed is the role of sub-micron and nanoscale particles. This includes (i) the fate of nanoscale biogenic suspended solid particles in the sludge, as well as (ii) the role of nanoscale dewatering aids to improve sludge conditioning. It is generally well known that due to their extremely smaller size, the biogenic sub-micron and nanoscale particles are not effectively captured in low shear (e.g. <1500 G) centrifuge devices, and require high shear (e.g. 2000 to 4000 G) centrifuging for effective dewatering.
The present invention is directed to the use of nanoscale additives during polymer-aided dewatering to increase the dewatering efficiency compared to traditionally used polymer-only dewatering processes. The use of nanoscale additive increases the percent solids in the dewatered cake compared to polymer-only treatment. Further, when nanoscale additives are added, the centrifugal force (shear) required to obtain high percent solids can be significantly reduced. In addition, the optimum polymer dose required for effective dewatering is also significantly reduced when nanoscale additives are added for dewatering.
Although it has been known that addition of fine particles can improve dewatering during polymer-aided dewatering, the fine particles used for this application are often inert substances (e.g. fly ash and cement kiln dust, which are micron scale particles) used to lower the specific resistance of dewatering by forming more porous and incompressible cake structure during dewatering. Hence, these fine particles are referred to as “skeleton builders” (See Zall, J., Galil, N. and Rehbun, M. 2001987. Skeleton Builders for Conditioning Oil Sludge. J. Water Pollution Control Federation 59(7):699-706 and 11. Benitez, J., Rodriguez, A. and Saurez, A. 1994. Optimization Technique For Sewage Sludge Conditioning with Polymer and Skeleton Builders. Water Research 28(10):2067-2073.). However, for effective dewatering using skeleton builders a large amount of fine particles (sometimes as high as 5 kg of skeleton builders for each kg of sludge) may be required. This generates significantly large amounts of dewatered solids, rather than lowering the solids required to be hauled as found with the nanoadditive-aided dewatering. Furthermore, skeleton builders are used as inert additives with no known charge specific mechanism of action with the sludge constituents. Secondly, skeleton builders are not known to lower the extent of centrifugation required for dewatering. The present invention concerns development of a nanoparticle additive that can improve the sludge dewatering process by increasing the solids content, lowering the centrifugation, and lowering the optimum polymer dose.
In one study, researchers from National Taiwan University of Science and Technology evaluated the use of alumina sub-micron sized particles for sludge dewatering (See Wang, Z. S., et al. 2007. Sludge Conditioning by Using Alumina Nanoparticles and Polyelectrolyte. Water Science and Technology. 56(8): 125-132). They reported improvement in dewatering performance (reduction in CST and specific rate of filtration) through the use of these additives when used along with cationic polymers. However, the sub-micron sized additives used by these researchers are significantly larger (139.5 nm to 326.4 nm) than the ones used in our work. Further, their data conflict with our findings that larger size nanoparticles did not improve dewatering significantly.
Also, the paper industry has used nano (colloidal) silica to improve retention and drainage characteristics during pulp and paper manufacturing (See Andersson, K. and Lindgren, E. 1996. Important Properties of Colloidal Silica in Microparticulate Systems. Nordic Pulp and Paper Research Journal. 11(1): 15-21). However, these additives have not been used for wastewater sludge treatment. Further, the nanosilica materials used by the paper industry are negatively charged. Use of negatively charged nanosilica did not improve sludge dewatering in our studies.